The invention relates generally to optical fibers and, more particularly, to optical fibers providing lateral viewing for use during medical procedures.
Optical coherence domain reflectometry (xe2x80x9cOCDRxe2x80x9d) is described in R. C. Youngquist et al, xe2x80x9cOptical Coherence-Domain Reflectometry: A New Optical Evaluation Technique,xe2x80x9d Optics Letters, Vol. 12, No.3, pp. 158-160, (1987). B. L Danielson et al, xe2x80x9cGuided-Wave Reflectometry with Micrometer Resolution,xe2x80x9d Applied Physics, Vol.26, No. 14, pp. 2836-2842, (1987), describes an optical reflectometer that uses a scanning Michelson interferometer in conjunction with a broadband illuminating source and cross-correlation detection. The first application of OCDR to diagnose biological tissue is noted in X. Clivaz et al, xe2x80x9cHigh-Resolution Reflectometry in Biological Tissues,xe2x80x9d Optics Letters, Vol. 1, No.1, pp. 4-6, (1992). The similar technique of optical coherence tomography (xe2x80x9cOCTxe2x80x9d) has been used for imaging with catheters, as disclosed in U.S. Pat. Nos. 5,321,501 and 5,459,570 issued to E. A. Swanson et al.
Both OCDR and OCT use optical data collected by a single mode optical fiber to determine the morphology, physical properties and location of various types of interspersed biological tissue. More particularly, a probe used in conjunction with either OCDR and OCT is typically comprised of an optical fiber having a head at its distal tip. Alternatively, the probe is formed by inserting an optical fiber concentrically into a thin-wall flexible hypodermic stainless-steel tube and fastening it therein with cement. A window in the tube allows light to pass to and from the head at the tip of the optical fiber. The probe is then inserted into the tissue or organ to be examined. Light emitted by the head of the optical fiber is reflected from the adjacent body of tissue or organ. The head then collects the reflected light, also known as xe2x80x9cback-scatteredxe2x80x9d light.
Using a Michelson interferometer in conjunction with techniques and apparatus discussed in the aforementioned references, the elapsed time necessary for each light ray to return to the fiber is calculated. From this data, the morphology, properties and location of the various tissue or organ elements that caused the back-scattered light are determined and an image generated to provide a real-time visual display of the body of tissue or organ being examined.
The real-time image provided by OCDR and OCT has been used to great advantage in conjunction with medical procedures. However, as a typical optical fiber can only emit light and gather back-scattered light along its axial centerline, it is limited to viewing straight ahead. A view transverse to the axial centerline of the fiber can only be obtained by turning or bending the head of the fiber perpendicular to its axial centerline, and this is often very difficult or even impossible in the close confines typically encountered during surgical procedures, e.g., in examining the sides of an artery or vein.
One solution to this problem is to mount a gradient refractive index (xe2x80x9cGRINxe2x80x9d) lens or a mirrored corner cube on the head of the optical fiber. Both of these devices deflect the emitted light at an angle transverse to the axial centerline of the optical fiber, and thus provide for lateral viewing. However, these apparatus add bulk to the head of the optical fiber. For example, the diameter of an optical fiber typically used in conjunction with OCDR and OCT is 90 microns (10xe2x88x926 meter), while the diameter of the smallest GRIN lens is 150 microns and that of the smallest mirrored corner cube is 125 microns. The use of either of the aforementioned optical devices thus renders some locations inaccessible and makes the optical fiber more difficult to maneuver. In addition, extremely small GRIN lenses and mirrored corner cubes are quite expensive, and very fragile. Their use thus adds to the cost of the probe, and renders it prone to malfunction.
As may be seen from the foregoing, there presently exists a need in the art for an optical fiber head that provides for lateral viewing without increasing the size of the fiber head. The present invention fulfills this need in the art, and does so without adversely affecting the reliability of the optical sensing probe and at significantly less cost than the prior art devices used for this purpose.
Briefly, the present invention is a modification to the head of a single mode optical fiber comprising the sensing probe of an optical heterodyne sensing system, such as an OCDR or OCT device, which enables optical sensing transverse to the axial centerline of the fiber. The optical fiber including the present invention is typically incorporated into a catheter, endoscope, or other medical device to determine the location, thickness, and morphology of the arterial walls or other intra-cavity regions during a minimally invasive medical procedure. The information is used to guide the sensing probe through the body as well as evaluate the tissue through which the sensing probe is being passed.
More particularly, a planar surface is formed at the end of an optical fiber, with the surface intersecting the perpendicular to the axial centerline of the optical fiber at an acute polishing angle. The surface is coated with a reflective material so that light traveling axially through the fiber is reflected at an emission angle relative to the axial centerline of the fiber, and passes through the side of the fiber. Alternatively, the planar surface can be left uncoated. In the latter case, total internal reflection occurs due to the differential in density between the core of the optical fiber and the adjacent tissue or fluid. To avoid internal reflection from the side of the optical fiber that occurs when the emission angle is too close to 90xc2x0, the polishing angle should be at least 51xc2x0 or, alternatively, no greater than 39xc2x0.
A portion of the emitted light is reflected by biological tissue and collected by the fiber head. Processing the back-scattered light using OCDR or OCT techniques and apparatus provides images of tissue located lateral to the optical fiber. The maximum or minimum polishing angle is determined primarily by the extent to which it is desired to deviate from a view normal to the side of the optical fiber, rather than by limitations arising from optical mechanics.
The present invention provides for viewing biological tissue transverse to the axial centerline of an optical fiber comprising a sensing probe, without increasing the size of the fiber head. It also achieves the foregoing without compromising the durability of the sensing probe or adding substantial cost to its manufacture.
The present invention also includes the method for modifying the optical fiber to obtain the aforementioned configuration. The end of the optical fiber is first cut at the proper polishing angle. The resulting planar surface is then polished with the circumferential buffer intact, rather that stripping the buffer before polishing as is usually done. The fiber is then coated with a reflecfive material using a standard technique such as sputter coating. Alternatively, the foregoing step may be omitted.
Next, the buffer is stripped from the end of the fiber. Stripping the buffer at this point also removes any reflective material that may have overlapped onto the buffer. Finally, the head is coated with a transparent protective material.